dual hub motor bike with a front direct drive, rear geared?

[lclarkberg]

Hi all. I am considering building a dual hub motor (2WD) bike with a direct drive motor on the front and a geared motor on the rear. Some background: my first electric bike was a Big Dummy longtail cargo bike with a Stokemonkey mid-frame motor. Because the Stokemonkey is geared down both with a chain going from the motor to the pedals and a chain going to the freewheel it has incredible hill-climbing power--it's like a power winch on wheels. It's slow going, but it could climb even the steepest hills on 600W. I could easily carry my 11-year-old daughter and four bags of groceries up the steepest hills in Ithaca.

I recently upgraded to a much more powerful 3KW bike with 10s1p lipos and a Crystalyte HS3540 motor. It's fast, but surprisingly not a good a hill climber. A quick trip to Justin's simulator shows that in order for this bike to put out maximum power up a hill I need to limit my load and maintain 20mph or so.

It occurs to me that I'd like my bike to have both a geared motor for hill climbing (maybe even with a 24" or 20" rim) and a direct drive motor for greater power on the flats. This would in effect give my bike two "gears", a low gear for the hills and a high gear for the flats. I might try two throttles at first to get a feel for it. Eventually I'd make a single throttle that runs the geared motor first and then shifts to the direct drive motor. I would put the geared motor on the back for better traction, and if the geared motor had a smaller rim the shorter profile could give my cargo bike more cargo space.

Most dual hub builds I've seen have both hub motors the same type. Has anyone out there tried mixing both types of motor? Any advice?

 

[motomech]

http://endless-sphere.com/forums/viewtopic.php?f=3&t=30652&hilit=motor+cute

http://endless-sphere.com/forums/viewtopic.php?f=2&t=33429&p=485910&hilit=recumbent+trike#p485910

http://endless-sphere.com/forums/viewtopic.php?f=2&t=26046&p=379211#p379211

http://endless-sphere.com/forums/viewtopic.php?f=6&t=36959

This will get your reading started.
There are a couple more threads if you scroll down the topics page.

 

[teklektik]

Hi all. I am considering building a dual hub motor (2WD) bike with a direct drive motor on the front and a geared motor on the rear.

Other way around - Use DD for booking on the flats in the rear - this gives good traction in front for steering, good traction in rear for power. Use geared in front for hill climbing - freewheel clutch will take it out of the picture for DD cruising, preserving your steering. Just switch off the front motor when you don't need it (see threads for various control options).

 

[dogman dan]

I tend to agree, put the gearmotor up front. If nothing else, because you already have a rear dd.

Something like a 12t mac front motor would be nice for grinding up the hills. Not sure if you can get those in a frontie.

 

[miuan]

I have a front 10t geared and rear DD motor that is about 7% faster than the front. up the hills and during takeoffs I engage both motors, and cruise on the rear only, up to 35-38mph. I get 40-43mph with both motors full throttle in the odd case when I need to keep up with faster car drivers.
My 2nd bike has a 10t rear geared motor only and does about 35 mph on flat no wind. I use both for commuting, no offroad riding.
I like both but the 2WD is just so much better. Given that I only take it if there are no stairs in the way, it's so bloody heavy. But the power reserve, thermal stability, propulsion redundancy and crazy uphill traction set it far apart of your conventional ebikes.
I run all these motors at 74V and 20-25A. I can be literally reckless on the 2WD but on the other bike, it takes some caution to keep it from overheating on long steep rides when the motor dissipates over 500W heat. Luckily I'm not fat enough to toast it.

 

[lclarkberg]

Thanks everyone for the quick responses. It took me a while to read all the previous posts. It looks like the advantages of 2WD was controversial a few years ago but now it's pretty well established that it's useful for certain applications and maybe even generally advantageous. For me the main advantage will be hill-climbing with a heavy load, sometimes as heavy as two adult passengers and sometimes on hills with a 20% grade. And I like the idea of redundancy. I'm tired of walking home after blowing a controller.

My goal is to create the ultimate car-replacement vehicle. One of the main uses of my car is to ferry kids (and occasionally adults) around town. So I need a bike that can do that reliably. I know I'll have succeeded when I can take my mom to her doctor's appointment on my bicycle.

I have a few DD and geared hub motors from E-bikekit so I can try different configurations and post about what I find out. I know their DD motors are 9C but I'm not sure what brand the geared motors are. I'm concerned that putting a motor on the front will cause a traction problem. One of my bikes' front hub motors slips on steep hills. But maybe a cargo bike will have less of a problem. It was useful to read that some people have had trouble with a front wheel DD motor slipping on turns.

Many previous 2WD posts assume running both motors all the time. On the flats I plan to run both motors upon takeoff, then run only the DD motor. On hills I plan to run both on takeoff and then mostly the geared motor. That's what makes sense to me at this point but maybe an actual road test will show some flaw in that reasoning. I'm glad to hear mluan's strategy is similar to this.

 

[MadRhino]

...now it's pretty well established that it's useful for certain applications and maybe even generally advantageous...

First yes, second no

It may be useful for certain applications, but generally NOT advantageous.

 

[miuan]

For the ultimate car-replacement vehicle, a STEEL cargo bike is the way to go. I have an aluminium bike (Kona Ute) because I need to be able to pick it up and carry up stairs daily, but if weight is no concern, go with the heaviest most rugged longtail frame you can get. Many have used Yuba Mundo with best of luck, and you can find a very detailed and overall excellent build thread here on ES.

You will also need custom wheel builds done with 10 to 12G spokes and bored rim eyelets using the strongest DH rims available. From my experience, I strongly recommend using Stan's rim strips and sealant. No matter what I did I never got a flat with them. Even cracked and bent a rim, sprayed the goo all around on my DH bike, but the strip still kept the air in my tyre.

Don't worry about front traction on a cargo bike. I've had zero problems on the Ute. Use the motor you already have up front, both DD and geared will be fine if you use enough torque arms and a tough fork. Give the rear DD 50% more amps than upfront. As the rig becomes heavy, you realize the extra 2kg don't make much difference, so you can also make a dual DD rig like I did a year ago. I also used it to carry my 100kg brother on the odd alcoholic occasion, and it did fine.

The strategy should be both motors on anytime you need serious power. You want to share the load between motors so you rarely slip out of the motors' efficiency zone. About the only time you turn the front motor off is for cruising on flats, or when you need to be stealth. As stealth as one can be riding such a bike.